This invention relates to measuring the speed of a vehicle, and more particularly to a method of minimizing erroneous speed measurement due to the effects of vibration on a variable reluctance speed sensing mechanism.
The speed of a vehicle is typically measured for display and control purposes by installing a variable reluctance sensor in proximity to the teeth of a rotary wheel such as a transmission output gear. When a gear tooth passes the sensor, the magnetic flux through the sensor changes, producing a corresponding quasi-sinusoidal pulse on an output signal of the sensor, and a signal processing circuit responsive to the sensor output signal provides a speed signal to an electronic control module (ECM). As a practical matter, the signal processing circuit may be located within the ECM, as a sub-block between the sensor input pins and a CPU that measures the signal frequency for purposes of calculating vehicle speed. With any individual sensor, the output signal amplitude varies with the detected change in flux, but the amplitude can also vary from sensor to sensor due to manufacturing tolerances and installation variations. Accordingly, the objective of the signal processing circuit is to provide maximum filtering of the sensor output signal for noise rejection, while still recognizing legitimate signals produced by a minimum output sensor, and to provide a corresponding digital speed signal to the ECM.
The above-described approach works well under most conditions, but is subject to erroneous speed indications when the transmission output gear is unloaded and stationary and the engine speed is relatively high. Under these conditions, the transmission output gear tends to vibrate both rotationally and/or radially, which can produce significant and psuedo-random magnetic flux variations in the variable reluctance sensor, particularly if a gear tooth happens to be directly aligned with the sensor. Unfortunately, the sensor output signal produced under these conditions is frequently indistinguishable from a normal sensor output signal; that is, the amplitude and frequency of the vibrational signal can fall within the expected amplitude and frequency ranges of a legitimate sensor output signal. Accordingly, what is needed is a method of measuring vehicle speed that recognizes a legitimate sensor output signal, but that is insensitive to sensor output signals produced by gear tooth vibration.
The present invention is directed to an improved method of measuring vehicle speed based on the output signal of a variable reluctance sensor responsive to the teeth of a transmission output gear, wherein the sensitivity of a signal processing circuit receiving the sensor output signal is adjusted in dependence on the mode of operation of the vehicle so as to reduce sensitivity to output signals produced by gear tooth vibration. According to the invention, the signal processing circuit only passes portions of the sensor output signal that have an amplitude greater than a predefined threshold and a frequency less than a predefined frequency, except when the ECM identifies a condition of potentially erroneous speed sensing characterized by substantially stationary vehicle operation with the engine decoupled from the output gear and an engine speed in excess of a calibrated threshold. Under the identified condition, the ECM reduces the sensitivity of the signal processing circuit by raising the predefined threshold and lowering the predefined frequency so that only large amplitude, low frequency signals are passed.